The use of gelatin, a hydrophilic colloid, as the binding agent or vehicle in photographic silver halide emulsions and other components such as couplers, is well known. Such loaded layers do not have good abrasion resistance and must therefore usually be coated with an overcoat layer. It has been found that overcoat layers, containing gelatin and mixtures of gelatin and other hydrophilic colloids are suitable for this purpose. The addition of certain materials to both the silver halide layer and the overcoat layer improves resistance to abrasion, particularly when processed in high speed automatic processing equipment such as described in U.S. Pat. Nos. 3,147,090 and 3,025,779. The use of high-speed, high-temperature processing of these materials, however, results in a reticulation problem in the photographic element.
Reticulation or extensive layer buckling in an irregular pattern is a serious problem and occurs when the uppermost layer of a photographic element is a gelatin-containing layer that has a lateral swell coefficient greater than the lower gelatin-containing layers. Since the top layer is the only one not confined at both boundaries, it has more freedom to swell laterally. Thus, any gelatin based coating is inherently prone to reticulation. Large lateral swelling tendencies are produced when layers are coated in a manner under conditions that include high drying temperature, high wet coverage, and short low temperature chilling. The web temperature, in the dryer of the coating machine, controls the triple helix crosslink formation of gelatin. In general, the higher the temperature, the less the number of crosslinks formed and the higher is the differential in the lateral swell. The ratio of the gelatin vs non gelatin material in the underlying layers, relative to the top layer also contributes to lateral swell. In order to minimize the lateral swell differential, low ratio is desired in the top layer and a high ratio is desired in the underlying layers.
When gelatin-containing layers are rewet, such as during processing, the gelatin structure in each of the layers swells as it absorbs aqueous processing solutions. Usually, since the uppermost layer has less non-swelling material per volume of coating than does the lower layer, it can absorb more water and swell more. Most of this swell is in the vertical direction. However, when the horizontal or lateral swell occurs, the tendency for each of the layers to swell to different lengths causes large swelling stresses between the uppermost layer and the contiguous lower layer and a buckling of the surface of the uppermost layer occurs when these stresses are relieved during drying. The differential horizontal swell between layers can be minimized if the proper crosslinked structure of the gelatin has been developed during the coating and drying operation. The structure of the dried, coated element is strongly affected by the rate of drying and the temperature of drying subsequent to the coating operation as mentioned before. During most coating operations the web temperature is increased along the length of the drier, in order to dry the coated element completely. The gelatin concentration in the coated element also increases along the length of the drier as a result of drying. The web temperature at which the gelatin concentration is between 15% and 60% is critical to the formation of the triple helix structure, which affects the amount of reticulation. As the amount of water load in the coating is increased, the critical gelatin concentration is achieved at the later sections of the drier, thus increasing the web temperature in this critical region. Because of this, products coated with high water loads, are more prone to reticulation, in operations where the drier capacity is limited. However, reducing the amount of water, can cause other problems, such as increase in viscosity, or increase in the shear thinning propensity of the coating solutions. As the drying time, during manufacture, is extended, the risk of reticulation is lessened. However, this slows down the coating speed resulting in a more expensive process and reduced production capacity of the photographic element in machines which are limited in drying capacity. Under these constraints it is desirable to manufacture the product without having to increase the dryer capacity, and without a reticulation problem.
While reticulation occurs mainly in gelatin containing multilayer elements, that is, elements having at least one pair of contiguous gelatin containing layers, it can also occur in single layer elements where a single gelatin layer is coated on a relatively nonswellable support.
The elements of the coating operation that contribute to this problem are the overall high water content of the coating composition and the low gelatin to non-gelatin content of the lower layers compared to the upper most gelatin containing layer of emulsion side of the photographic element in conjunction with high speed coating and drying operations. For example, commonly owned U.S. Ser. No. 09/299,395, filed Apr. 26, 1999 and U.S. Ser. No. 09/299,548, filed Apr. 26, 1999, disclose a photographic element which is prone to severe reticulation if it is coated above a speed of 900 ft/min. It is desired to manufacture this material at speeds higher than 900 ft/min, in order to reduce the unit manufacturing cost, without installing extra dryers in the machine.
No prior art uncovered discloses a processing solution permeable non-gelatin overcoat which overcomes the problems discussed above. It is therefore highly desirable to provide an imaging element comprising a protective overcoat layer that reduces reticulation of the gelatin containing layers without significantly reducing the rate of reaction of the developer with the underlying emulsions.